In order to colour keratinic fibers, in general, either direct dyes or oxidation dyes are used. Although intensive colours can be obtained with good fastness properties using oxidation dyes, development of the colour in general occurs by the action of oxidizing agents such as H2O2, for example, which in some cases can result in damage to the fibers. Furthermore, some oxidation dye precursors or specific mixtures of oxidation dye precursors may have a sensitizing effect on some people with sensitive skin. Direct dyes are applied under kinder conditions. However, they suffer from the disadvantage that the colours frequently do not have satisfactory fastness properties.
The person skilled in the art categorizes direct dyes into different dye classes as a function of the desired colour result. As an example, direct dyes which are known in the art belong to the nitro dye, anthraquinone dye, azo dye, triarylmethane dye or methine dye classes. All of these classes of dyes should comply with a specific specification profile for use in cosmetics. Thus, direct dyes should provide an intensive coloration and have fastness properties which are as good as possible. The resulting colour obtained with direct dyes should be affected by the environment as little as possible, i.e. the dyes should, for example, have good wash fastness, light fastness and rubbing fastness. Chemical actions to which the keratinic fibers could be exposed following the dyeing process (such as permanent waving, for example) should also alter the resulting colour as little as possible.
In order to obtain a lightening effect at the same time as a colour, the direct dyes should, where possible, also be compatible with the oxidizing agents normally employed during a bleaching procedure (for example hydrogen peroxide and/or persulphates).
In order to ultra-lighten dark hair, not only hydrogen peroxide alone, but also a combination of hydrogen peroxide and persulphates (for example ammonium persulphate, potassium persulphate and/or sodium persulphate) is used. If, then, dark hair is ultra-lightened in one step and simultaneously dyed to a bright shade, then the use of a mixture of hydrogen peroxide, persulphates and a direct dye is of advantage. Although the person skilled in the art will be aware of many intensively colouring direct dyes for colouring hair, he will only be aware of a very limited selection of dyes which can tolerate the highly oxidative conditions occurring with a mixture of the oxidizing agents mentioned above without decomposing. In addition, the dyes known in the prior art which are stable to oxidation suffer from severe disadvantages as regards their other fastness properties.
For simultaneous colouring and ultra-lightening of hair, then, there is still a need for dyes with a high stability as regards strong oxidizing agents. Even under these extreme application conditions, these dyes should not lose their positive fastness and colouring properties.
It has been shown that bright and intensive colours can in particular be produced with cationic direct dyes. Cationic dyes are frequently distinguished by a particularly high affinity for keratinic fibers, which can be attributed to the interaction of the positive charges of the dyes with negatively charged structural components of the keratinic fibers. Thus, frequently, particularly intensive colours can be obtained with cationic dyes.
Exemplary representatives of monomeric cationic azo dyes which have been known in the prior art for a long time are Basic Orange 31 (alternative name: 2-[(4-aminophenyl)azo]-1,3-dimethyl-1H-imidazolium chloride, CAS-No. 97404-02-9) and Basic Red 51 (alternative name: 2-[((4-dimethylamino)phenyl)azo]-1,3-dimethyl-1H-imidazolium chloride, CAS-No. 77061-58-6).
Both dyes colour keratinic fibers with an excellent colour intensity in the orange to red range of nuances. There is still a need for direct dyes which are as optimally compatible as possible with these two dyes.